Assessment of near-field rock mass fracturing around a potential nuclear fuel waste repository in the Canadian Shield

Abstract Atomic Energy of Canada Limited (AECL) is developing a concept for the permanent storage of Canada's nuclear fuel wastes. As of 1992, the concept was based on in-floor emplacement of waste canisters within an engineered repository constructed at depths of 500–1000 m in plutonic rocks of the Canadian Shield. Canadian law requires that the Federal Environmental Assessment Review Office (FEARO) undertake a formal review of the concept before AECL proceed with site selection and design. In preparation for this upcoming review, the federal ministry which is responsible for the environment, Environment Canada, commissioned a small panel to undertake an overview of the concept. The sole purpose of the panel's work was to identify issues of potential importance to the safety and acceptability of the storage concept, so that the future review process could be appropriately focused on these key issues. Although fracture mechanics research has made much progress in understanding rock mass failure in terms of fundamental fracture initiation, propagation and interaction processes, there is still no generally accepted method by which to evaluate the actual fracturing response of a real rock mass in situ. Nevertheless, public review of the repository concept must address the issue of potential rock mass fracturing around the underground openings. The challenge is to do so despite the lack of a generally agreed methodology, in a manner which is practical within tight budgetary limits, acceptable in the context of normal engineering practice, and sensible from a fracture mechanics perspective. The key question is whether simple rock engineering analyses, based on conventional stress-strength comparisons, can be used to assess the potential for near-field fracturing of the rock mass. The goal of the work reported in this paper was to undertake a realistic but limited pre-feasibility level engineering evaluation of the potential for near-field fracturing of the rock mass, particularly in the vicinity of the canister emplacement holes. The approach chosen was based on simple parametric modelling using boundary element analyses. Input parameters were selected to reflect a range of conditions appropriate to batholiths of the Canadian Shield. The results, based on conventional rock engineering stress-strength analyses, were evaluated within the context of current fracture mechanics research views and of engineering judgement derived from experience with rock mass fracturing around deep underground openings in the mines of the Canadian Shield. Results indicate that stress concentration effects will cause fracturing beneath the room floors, and experience suggests that rock damage will be mainly in the form of horizontal slabbing. With time, rock damage may extend over the full 5 m depth of the emplacement holes. These findings suggest that the concept of in-floor storage of the waste canisters must undergo detailed critical review before being accepted as an inherent part of the AECL disposal concept.